The leaders of more than 20 world cities are meeting in London to swap ideas on combating climate change.

Berlin's investment in solar cells, Mexico City's taxi fleet upgrade, and Toronto's use of lake water cooling for its buildings are all on the agenda.

London Mayor Ken Livingstone, who is hosting the event, says that cities have a special responsibility to curb emissions, and special opportunities.

Cities consume about 75% of global energy production.

The World Cities Leadership Climate Change Summit brings together representatives from a diverse range of cities, including Beijing, Delhi, Sao Paulo, Stockholm and Kingston.

Climate change is our biggest problem, and cities have special issues

Nicky Gavron, London's deputy mayor

On the agenda will be dealing with the impact of climate change as well as trying to reduce emissions.

"Everyone has a handful of good examples of dealing with impact and reducing greenhouse gas emissions," said London's deputy mayor, Nicky Gavron.

"Climate change is the biggest problem facing us, and cities have special issues such as the heat island effect and flash floods," she told the BBC news website.

"Seventy-five percent of worldwide energy use is in buildings, so we have a special responsibility; but we also have special opportunities because of the concentration of buildings and the nature of the labour market."

Green shoots

The Climate Group, the UK-based organisation that seeks to advance government and business leadership on climate change, released a report on the eve of the meeting setting out 15 case studies of cities which have met with some success in reducing emissions.

Among the examples cited are

Berlin, where 75% of new buildings have to include solar panels in their design

Mexico City, which plans to replace 80,000 taxis with low-emissions vehicles by 2006

Chicago, which is encouraging the use of rooftop gardens to keep buildings cool

Copenhagen, where one-third of the population cycles to work.

One of the most imaginative low-carbon projects comes from Toronto, where cold water from the lower depths of Lake Ontario is circulated around the city.

More than 30 buildings in the city centre now use the Deep Lake Water Cooling Project rather than conventional air conditioners.

London itself will present details of its congestion charging scheme, under which car drivers have to pay a sum of £8 (US$14) for each journey into the city during working hours.

Mr Livingstone says that carbon dioxide emissions within the congestion charge zone have fallen by 19% since it was introduced.